The present invention relates generally to a process for the gasification of carbonaceous solids and a fluidised bed reactor for carrying out such a process.
A known form of fluidised bed reactor for the gasification of carbonaceous solids comprises at least one nozzle for the injection of exothermic and endothermic gasification agents into the interior of the reactor. The nozzle is provided with at least two mutually coaxially disposed pipes which thus define at least one annular opening. The outer pipe, which is provided for supplying at least predominantly endothermic gasification agent, is thus disposed around the inner pipe which is provided for supplying at least predominantly exothermic gasification agent. The mouth opening or discharge orifice of the inner pipe projects further into the interior of the fluidised bed reactor than the mouth opening or discharge orifice of the outer pipe, while a region of increased temperature is formed in front of the orifice of the inner pipe, within the reactor, due to the reaction of the exothermic gasification agent with combustible substances.
Generally, the reactor has a plurality of such nozzles which are distributed around the periphery thereof, and the nozzles may possibly be arranged at two or more levels which are vertically spaced from each other.
More specifically, a reactor of the aboveindicated nature is disclosed in `Freiberger Forschungshefte` A69, 1957, pages 10 and 11. The supply of carbon dioxide in the outer pipes of the nozzles is provided to reduce the temperature of the fluidised bed in the region where the oxygen is introduced, that is to say, the discharge orifices, in order in that way to reduce slag deposits. The problem involved in the formation of such slag deposits or baked-on formations is also dealt with in German laid-open application (DE-OS) No. 31 43 556 which discloses a reactor provided with a nozzle which has at least three coaxial pipes. The middle pipe is provided for supplying the carbonaceous materials which are to be gasified. The exothermic gasification agent is supplied through the annular space or opening which is defined by the inner pipe and the middle pipe, while the predominantly endothermic gasification agent is introduced into the interior of the reactor through the annular opening defined by the middle pipe and the outer pipe. The arrangement in that reactor may be such that the pipe which externally delimits the duct for the exothermic gasification agent may terminate at a spacing in the axial direction, upstream or in front of the other two pipes. The spacing between the free end of the outer pipe and the free end of the other two pipes however is only a few millimeters. The formation of baked-on deposits is intended to be prevented by virtue of the discharge end of the outer pipe for the endothermic gasification agent being reduced in the direction of flow therethrough to a very small wall thickness in order in that way to give a correspondingly small end surface so as to afford the minimum possible area for deposit thereon of the particles of ash which are to be found in the interior of the reactor. In addition, in the above-indicated German laid-open application, there is a discussion regarding the dependency between the formation of baked-on deposits and the relative speeds at which the agents in the mutually coaxial pipes are injected into the interior of the reactor. Thus, the flow speed of the endothermic gasification agent is to be from 70 to 85% of the speed at which the exothermic gasification agent flows into the interior of the reactor. The above-described steps are intended to provide regions which have different levels of oxygen concentration, in the area in front of the discharge orifice of the nozzle, in order thereby to reduce the speed at which the solid carbon of the individual particles is reacted, while also seeking to reduce the extent to which sintering of the individual particles occurs.
The extent to which the arrangements disclosed in German laid-open application No. 31 43 556 are such as to provide the desired effect in the reactor described therein can be left in abeyance as there is in any case no possibility of transferring same to a fluidised bed reactor wherein the carbonaceous materials to be gasified are introduced into the interior of the reactor through a particular supply means which is independent of the nozzles for supplying the gasification agent, especially as such nozzles generally do not extend vertically upwardly into the reactor.
In the Winkler reactor which is disclosed in the publication first referred to above, the cooling action which is produced by the supply of carbon dioxide may have been sufficient to achieve the desired aim, as such a reactor was operated under normal pressure. However, modern fluidised bed reactors, more particularly high temperature Winkler reactors, are operated at increased pressures of 10 bars and more. The use of increased pressure serves to increase the rate of through-put, that is to say, the amount of coal which is to be passed through per unit of time. That presupposes a suitable increase in the amount of gasification agent, that is to say also the exothermic gasification agent. As a result, the specific thermal loading in the reactor is increased, which in turn gives rise to a greater danger of ash deposits and baked-on formations occurring. As indicated also in the prior art, such phenomena occur more particularly at the end faces of the nozzle pipes. That is in essence to be attributed to the fact that temperature peaks occur in the interior of the reactor, at a short spacing from the mouth opening of the nozzles which supply exothermic gasification agent, such temperature peaks occurring more particularly in the region in which the oxygen supplied first comes into contact with combustible substances, either solid particles or combustible gases. The heat which radiates from that high-temperature region acts on the end faces of the respectively associated nozzles so that solid particles with a high ash content, which are to be found in the high-temperature region, undergo softening and stick to the nozzle. In that way, deposits may be formed, the dimensions of which are finally of such a magnitude that the reactor has to be taken out of operation.